Tonal noise mainly originates from flow irregularity (non-uniform flow) that causes circumferentially varying blade forces and gives rise to a considerably large radiated dipolar sound (tonal noise) at the blade passage frequency (BPF) and its harmonics. In axial fans, axial blade forces are mainly responsible for the tonal noise. In centrifugal fans, a combination of axial, radial, and tangential blade forces and the interaction between the rotor and the housing are mainly responsible for the tonal noise. Although some fans operate in an environment where the flow is uniform, as schematically illustrated for an axial fan in FIG. 2A, in many instances, fans operate in a non-uniform flow, as schematically illustrated for an axial fan in FIG. 2B: this is the case, for example, of engine cooling fans that operate behind a radiator/condenser system or in the wake of inlet guide vanes.
Techniques to control fan noise can be classified into two main families: active control or passive control. Passive methods are principally based on the geometrical characteristics of the propeller and its environment to reduce the noise generation mechanisms (reduce fluctuating forces or minimize their acoustic effects). Passive techniques can be considered as preventive techniques. However, it is not always possible to apply such modifications, especially in case of confined environments, such as automotive engine cooling fans. In such cases, active techniques have been proposed. Active techniques are effective at low frequencies, where passive techniques (such as using absorbing materials) are inefficient. Active techniques use the destructive interference between two waves to attenuate the noise. This is done by a secondary noise generated by a secondary source (loudspeaker for example) that interferes with the fan's primary noise. Active techniques can be considered as corrective techniques.
A number of solutions for controlling tonal noise in axial fans have been proposed. U.S. Pat. No. 6,375,416 presents a technique and an apparatus based on sinusoidal circumferential variation of the tip clearance to create an unsteady pressure field opposite in phase with respect to the primary unsteady pressure field, thus reducing tonal noise. The proposed technique is based on sinusoidal variations of the inner surface of the shroud. U.S. Pat. No. 5,692,702 describes a method as well as a system to control tonal noise generated by a ducted-rotor. The method relies on the introduction of upstream or downstream flow distortions to create an anti-sound opposite in phase with respect to the primary tonal noise. An acoustic signal from one or more microphone arrays provides information to adjust each circumferential modal component of the flow. Two methods for producing the distortions are proposed. The devices are mounted in a circumferential array on the duct wall and consist of either 1) nozzles actively exhausting or ingesting controlled amount of air or 2) rods with actively controlled protrusion into the flow. However, for the subject matter described in this patent, every modal component must be adjusted.
FIG. 1A schematically illustrates an adaptation of another prior art solution. A number of cylindrical rods 2A were mounted on a rotatable ring 4. Turning the ring 4 allowed for adjusting the phase of the control mode so that a reduction at the BPF was achieved when the two modes were out of phase. However, the wakes generated by the rods 2 are salient, leading to a high harmonic content rate of the unsteady lift. Thus, the high harmonic content rate can lead to amplification of higher acoustic tones when attempting to control tonal noise at the BPF.
Therefore, there is a need for a passive method and apparatus for controlling a tonal noise which does not significantly amplify higher acoustic tones. There is also a need for a passive method and apparatus for controlling a tonal noise which can be used in a confined environment.